1. Technical Field of the Invention
The present invention relates to imaging based on the ASL (Arterial Spin Labeling) technique, ASL imaging, capable of providing images of perfusion (tissue blood flows) or blood vessels non-invasively without administrating a contrast medium to the subject. More particularly, the invention relates to an MRI (Magnetic Resonance Imaging) apparatus and the ASL imaging technique capable of performing the ASL imaging that reduces errors induced by a time needed for a blood flow to pass through the path from the label region to the tissue of interest in the imaging region (that is, transit delay time), which is intrinsic to the slice-selective ASL technique.
The ASL technique referred to herein means the general spin labeling technique in the broad sense.
2. Related Arts
Magnetic resonance imaging is a technique involving magnetically exciting nuclear spins in a subject laid in a static magnetic field using a high frequency signal at the Larmor frequency, and obtaining images from excitation-induced FID (Free Induction Decay) signals or echo signals.
The spin labeling technique for evaluating perfusion of tissue, the so-called ASL technique, has been known as one category of magnetic resonance imaging. The ASL technique is used to provide images of blood veins or images of perfusion (tissue blood flows) reflecting microcirculation of the subject without administrating a contrast medium to the subject, that is, non-invasively, on which studies have been conducted actively in recent years. in particular, clinical applications are being developed chiefly for cerebral blood flow (CBF) of the head, and quantification of a blood flow volume is being enabled.
The ASL technique is broadly divided into the continuous ASL (CASL) technique and the pulsed ASL (PASL) technique (referred to also as the dynamic ASL (DASL) technique). The CASL technique is used to apply a large continuous adiabatic RF wave, by which a spin state in the blood vessel is labeled (magnetized) at a given point, and a change of the signal after the bolus of the labeled blood reaches the imaging slab (observation plane) is subjected to imaging. On the other hand, the PASL technique is used to apply a pulsed adiabatic RF wave, by which the magnetization in the blood vessel is constantly varied, and the tissue persistently susceptible to the magnetized blood flow is subjected to imaging, thereby enabling perfusion of this tissue to be evaluated. The PASL technique can be performed in a relatively easy manner with a clinical MRI apparatus.
Generally, two images are generated in the ASL imaging: images in a control mode and in a label (tag) mode. The image data obtained in each of the tag mode and the control mode is subjected to a pixel-by-pixel difference operation between these two images. An ASL image indicative of information of inflowing blood to the imaging slab, that is, microcirculation, can be thus obtained.
To be more specific, according to the ASL imaging, signals from the stationary tissue are generally cancelled and signals from the spins flowing into the capillary bed are obtained as a blood flow image. Hence, a difference image is obtained by computing a difference between a tag image taken after waiting for a certain time until the artery labeled in the tag region flows into the tissue of interest in the imaging region and a control image taken under the same conditions as those of the tag image without the labeling, and the difference image thus obtained is used as a blood flow image, that is, an ASL image.
Recently, there have been proposed various techniques of slice-selectively labeling an upstream artery outside the range of the slab to be imaged. Of these techniques, Article(1), Kwong K K, Chesler D A, Koff R M, Donahue K M et al.: MR perfusion studies with T1-weighted echo planar imaging, Magn Reson Med 1995; 34:878-887, and Article(2), Tokunori Kimura: Modified STAR using asymmetric inversion slabs (ASTAR)-hou niyoru hi-shinjyuu ketsuryuu imaging, JJMR (Japanese Journal of Magnetic Resonance in Medicine) (Nichijii-shi) 2001; 20(8), 374-385, are known as techniques for the PASL technique. On the other hand, for example, Article(3), Alsop D C, Detre J A: Reduced transit-time sensitivity in non-invasive magnetic resonance imaging of human cerebral blood flow, J.Cereb.Blood Flow Metab. 1996; 16, 1223-1249, is known as a technique for the CASL technique. These techniques are leading the SS-ASL (Slice Selective-ASL) to medical applications.
FIG. 1 schematically shows pulse imaging by the SS-ASL technique. As is shown in the drawing, in each of the control mode and the tag mode, a slice-selective pulse (SS-pulse), comprising an RF pulse and a slice gradient magnetic field Gs, is applied to the subject, and scan is performed to the imaging region after waiting for a certain inversion time T1. In FIG. 1, a solid line indicates the waveform of the slice gradient magnetic field Gs in the control mode and a broken line in the tag mode.
Normally, a living body has a characteristic that, in a path from the large blood vessel to the peripheral artery, the velocity of a blood flow flowing in the path is reduced with an increase of an average capacity of the blood vessel per unit volume. For this reason, a time needed for the blood flow to pass through the path from the tag region used for labeling to the tissue of interest in the imaging region (referred to as the transit delay time, abbreviated to Td) is not negligible in many cases.
In a case where the Td time cannot be neglected in comparison with the T1 relaxation (longitudinal relaxation) of labeled water (that is, when Td>T1), the labeled blood cannot reach the tissue of interest within the TI time, a time period from labeling to imaging of the upstream arterial blood, while maintaining sufficient longitudinal magnetization for imaging. The blood flow in the imaging region is thus under evaluated. Because the slice-selective ASL (SS-ASL) cannot, in principle, label a blood vessel within the imaging slab, this influence becomes particularly significant when the entire brain is to be covered by multi-slice imaging. Even in such a case, Article(4), Tokunori Kimura et al.: Arterial Spin Labeling imaging niokeru kyokusho nouketsuryuuryou no teiryouka kanben-hou-Pulsed ASL-hou niokeru simulation to cold-xenon-CT CBF tono soukan, JJMR (Nichijii-shi), 2002;22(3): 111-124, reports that an ASL image with satisfactory accuracy can be obtained by the SS-ASL with a normal subject whose blood flow velocity is high. However, in cases like the cerebral infarction where the blood flow velocity is reduced or where the Td time is extended due to a collateral vessel, the blood flow cannot reach the region of interest in the imaging region within the TI time, 1 to 2 sec. after the labeling. The flow is thus often under evaluated, which is reported as an issue of concern.
In order to reduce the influence of the Td time, a technique of capable of labeling the blood within the imaging slab has been proposed. This is a technique used to selectively label the spins in the flow at a given velocity or higher, and is proposed in Article(5), Norris D G et al., Velocity Selective Radio frequency Pulse Trains, J. Magn. Res. 1999; 137, 231-236. The result when this technique was applied in generating an ASL image of the brain of a normal subject was reported by E. Wong et al. in the 2002 meeting of the ISMRM as is described in Article(6), Wong E C et al.: Velocity Selective Arterial Spin Labeling, Abst Int Magn Res Med 2002; p621.
The ASL imaging technique according to Article(5) and Article(6) above is a technique (Velocity-Selective ASL, abbreviated to VS-ASL) used to label a blood vessel in proximity to the vascular bed within the slab where the velocity is low, using a velocity-selective pulse capable of labeling a blood flow flowing at a certain velocity value or higher.
FIG. 2 shows a schematic pulse sequence according to the VS-ASL technique. As is shown in the drawing, in the case of the VS-ASL technique, non-slice-selective 90° (x)−180° (y)−90° (−x) RF pulses are applied as the velocity-selective pulse (VS-pulse) in the control mode. On the other hand, in the tag mode, a pulse group, formed by adding a velocity encode pulse train, which is an MPG pulse, to the 90° (x)−180° (y)−90° (−x) RF pulses, is applied as the velocity-selective pulse (VS-pulse) This provides the control image and the tag image in the control mode and the tag mode, respectively, and a difference between the two images is computed. A difference image thus obtained is used as an ASL image. In this ASL image, spins having a flow velocity v=Venc (the flow velocity at which spins, having undergone transition to the transverse magnetization by the velocity encode pulse train, are rotated by 90°) or higher are excited (saturated). Because magnetization (Mz) of spins at v>Venc takes place only on the one side (in the control mode) according to this VS-ASL technique, hereinafter, this technique is referred to as the VS-ASL technique of a unipolar-VENC type (abbreviated to UVS-ASL technique).
The UVS-ASL technique, however, has unsolved problems as follows.
(1) A first unsolved problem is low sensitivity. To be more specific, the labeling effect of the magnetization Mz is increased by two times in the conventional SS-ASL technique, whereas the sensitivity is increased by one time or less in the UVS-ASL technique, which is lower than the sensitivity in the SS-ASL technique.
(2) A second unsolved problem is an artifact. To be more specific, in the UVS-ASL technique, there occurs unbalanced spoiling of the transverse magnetization components caused by setting MPG=0 in the control mode, that is, phase errors that result in a second-order spatial distribution induced by gradient magnetic field components in directions other than the main magnetic field direction, and the phase errors appear as artifacts remaining in the stationary tissue (background) where no flow should be present in an ASL image, thereby deteriorating the image quality.
(3) A third unsolved problem is extension of an imaging time. In general, because the ASL technique involves the averaging processing, plural sets of data are acquired and subjected to averaging. In other words, a pulse sequence of one cycle is applied repetitively a plurality of times at regular intervals. When the averaging processing is adapted to the UVS-ASL technique, because the UVS-ASL technique is non-slice-selective, it is necessary to secure a sufficiently long recovery time for the magnetization in blood to flip back to the longitudinal magnetization after a pulse sequence of one cycle is applied and before a pulse train in the following mode is applied. This results in a problem that a scan time is extended. In particular, in a case where RF transmission is performed using a whole body coil, because region including the heart is also excited by the RF pulses, the scan (imaging time) is extended.